1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof, and more particularly, to a fixing device for fixing a toner image on a recording medium, and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charging device uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may include a pair of looped belts or rollers, one being heated by a heater for melting toner (hereinafter referred to as “fixing member”) and the other being pressed against the fixing member (hereinafter referred to as “pressing member”). In a fixing process, the fixing member and the pressing member meet and press against each other, forming a so-called a fixing nip through which a recording medium is passed to fix a toner image thereon under heat and pressure.
To extend product life of the fixing member, which is subjected to constant heat and pressure, changing the position of the pressing member is proposed. More specifically, an eccentric cam is used to change the position of the pressing member between a pressing state and a pressure-free state by rotating the eccentric cam so that the pressing member is not always in contact with the fixing member. In this configuration, a single rotation of the eccentric cam in a predetermined direction changes a pressing state of the pressing member from a pressure-free state to a pressing state, or the vice versa.
Although advantageous, a direction of moment exerted on the shaft of the eccentric cam changes as the position of the pressing roller is switched from the pressing state to the pressure-free state, thereby causing surfaces of gear teeth contacting each other and/or surfaces in a joint portion where the camshaft and a drive shaft meet to be reversed, colliding with opposite surfaces, and producing undesirable noise such as impact noise.
To prevent such noise, a mechanism such as a damper or a one-way clutch that transmits force from a driver while locking the force from the camshaft has been proposed. However, such a mechanism is generally expensive.
In another example of changing the position of the pressing member, a plurality of pressure switching cams and a plurality of arms biased by a plurality of corresponding springs are provided to both lateral end portions of the pressing roller. Rotation of a pressure release lever causes the plurality of the pressure switching cams to rotate. In this configuration, the arms separate from the pressing roller against the pressing force of the springs in accordance with the position of the pressure switching cams, and a desired fixing pressure can be selected from a plurality of pressures accompanied by the plurality of springs. Accordingly, a pressing force of the pressing roller is changed to accommodate various types and thickness of recording media sheets.
Disadvantageously, this configuration tends to be complicated, with an increase in the number of parts such as multiple arms, springs, and cams required.